1. Field of the Invention
The present invention relates to a CMOS image sensor and a fabricating method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing diffused reflection that occurs due to the presence of metal lines by providing an opaque layer on a device isolation layer between microlenses.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device that converts an optic image to an electric signal. In a CCD (charge coupled device) image sensor, a plurality of MOS (metal-oxide-metal) capacitors are arranged close to one another to transfer and store electric charge carriers. In a CMOS (complementary MOS) image sensor, a plurality of MOS transistors corresponding to the number of pixels are fabricated by the CMOS technology using a control circuit and a signal processing circuit as peripheral circuits. Also, a switching system of sequentially detecting outputs using the MOS transistors is adopted.
The CMOS image sensor converts image information of a subject to an electric signal by using signal processing chips including photodiodes. Since an amplifier, an analog/digital (A/D) converter, an internal voltage generator, a timing generator, a digital logic, etc. can be integrated on each of the chips, the CMOS image sensor is advantageous in saving space, power and cost. Moreover, mass production of CMOS image sensors is enabled through a silicon wafer etching process that is cheaper than the fabricating process for CCDs, which includes a more complex process.
Thus, the CMOS image sensor has expanded into application fields such as digital cameras, smart phones, PDAs, notebook computers, security cameras, barcode detectors, toys, etc.
To increase the photosensitivity of the CMOS image sensor, many efforts have been made to increase the fill factor, which indicates a rate of number of pixels of a pixel area within a chip. However, a logic circuit part for signal processing takes up space and therefore limits the fill factor. Hence, a microlens forming technology has been introduced to increase the photosensitivity. A microlens condenses light by diverting a path of the light incident on an area that does not include a photosensing device, such as a photodiode.
An image of a subject may be transferred to the photodiode via the microlens. If a metal line within a device is in the way of a light path, diffused reflection occurs, which degrades image quality.
A method of fabricating a CMOS image sensor according to the related art is explained with reference to the attached drawing as follows.
FIG. 1 is a cross-sectional diagram of a CMOS image sensor according to a related art. Referring to FIG. 1, a first epitaxial layer (not shown) is grown on a semiconductor substrate 10. A red photodiode 11 is formed on the first epitaxial layer. A second epitaxial layer (not shown) is grown on the first epitaxial layer including the red photodiode 11. A green photodiode 13 is formed on the second epitaxial layer. A third epitaxial layer (not shown) is grown on the second epitaxial layer including the green photodiode 13. A blue photodiode 15 is formed on the third epitaxial layer. A trench for field isolation is formed on the third epitaxial layer. An STI (shallow trench isolation) layer 16 is then formed by filling the trench with insulator.
An insulating interlayer 17 is formed on the third epitaxial layer. A first metal layer (not shown) is formed on the insulating interlayer 17, and metal lines 23 are then formed by patterning the first metal layer. The process of forming the insulating interlayer 17 and the metal lines 23 is repeated several times to stack necessary metal lines 23. A device protecting layer 20, which may be made of nitride, is formed on the stacked insulating interlayer 17 to protect a device from moisture or physical shock. A microlens 22 is then formed on the device protecting layer 20.
In fabricating the CMOS image sensor according to the related art, if the metal line lies on a path of light having passed through the microlens 22, diffused reflection is caused and degrades image quality.
The related art method has the following problem. Since the device isolation layer, which may be nitride, is formed between the microlenses and is transparent, the diffused reflection degrades the image quality when the metal line lies in a path of the incident light via the device isolation layer.